In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) in addition to the early-stage voice service. 3rd generation wireless communication is followed by a 4th generation wireless communication which is currently being developed aiming at support of a high-speed data service of 1 gigabits per second (Gbps) in downlink and 500 megabits per second (Mbps) in uplink. Wireless communication systems are designed for the purpose of providing reliable communication to a plurality of users irrespective of users' locations and mobility. However, a wireless channel has an abnormal characteristic such as path loss, noise, fading due to multipath, an inter-symbol interference (ISI), the Doppler effect due to mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), etc., are techniques for supporting reliable high-speed data services.
An OFDM system capable of reducing an inter-symbol interference effect with a low complexity is taken into consideration as one of post-3rd generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols (where N is a natural number), and is then transmitted by being carried on N separate subcarriers. The subcarriers maintain orthogonality in a frequency dimension. In a mobile communication market, a standard is expected to be changed from a conventional code division multiple access (CDMA) system to an OFDM-based system.
The MIMO technique improves data transmission/reception efficiency by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Examples of the MIMO technique include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix depending on the number of Rx antennas and the number of Tx antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a layer or a stream. The number of layers is referred to as a rank.
For the purpose of data transmission/reception, system synchronization acquisition, channel information feedback, etc., there is a need to estimate an uplink channel or a downlink channel in a wireless communication system. Channel estimation is a process of recovering a Tx signal by compensating for signal distortion in an environment where a rapid change occurs due to fading. In general, channel estimation requires a reference signal known to both a transmitter and a receiver. The reference signal is also referred to as a pilot.
In the OFDM system, reference signals may be allocated by using two methods, i.e., a first method in which the reference signals are allocated to all subcarriers and a second method in which the reference signals are allocated between data subcarriers. The first method uses a signal (e.g., a preamble signal) consisting of only reference signals. The first method has a significantly improved channel estimation performance in comparison with the second method, but has a decreased data transmission rate. Therefore, the second method can be used to increase the data transmission rate. The second method may result in deterioration of the channel estimation performance since density of the reference signals is decreased. Therefore, it is required that the reference signals are properly arranged to minimize the deterioration of the channel estimation performance.
When the transmitter transmits a reference signal p and the receiver receives an Rx signal y, the Rx signal y can be expressed by the following equation.MathFigure 1y=h·p+n  [Math.1]
Herein, h denotes a channel on which the reference signal is transmitted, and n denotes thermal noise generated in the receiver.
In this case, the reference signal p is known to the receiver. The receiver can estimate the channel by using the reference signal p. The estimated channel h′ can be expressed by the following equation.MathFigure 2h′=y/p=h+n/p=h+n′  [Math.2]
Accuracy of the estimated channel h′ is determined according to n′. For the accuracy of the estimated channel h′, n′ has to converge to zero. Channel estimation may be performed by using a large number of reference signals to minimize an influence of n′. The receiver can compensate for the estimated channel to recover data transmitted by the transmitter.
Since antennas of a multiple antenna system respectively correspond to different channels, each antenna has to be considered in the designing of a reference signal structure. In a multiple antenna system, it is effective to use even power transmission in which each antenna has the same Tx power as much as possible. Even power transmission using multiple antennas can result in decrease in implementation cost and improvement in system performance.
However, when the reference signal structure is designed so that even power transmission is possible in the multiple antenna system, a reference signal overhead may be significantly increased. The reference signal overhead can be defined as a ratio of the number of subcarriers for transmitting the reference signal to the number of all subcarriers. When the reference signal overhead is great, there is a problem in that the number of data subcarriers for transmitting data in practice is decreased. This results in decrease in a data processing load and deterioration in spectrum efficiency. As a result, an overall system performance may deteriorate.
Accordingly, there is a need for a method and an apparatus for effectively transmitting a reference signal in a multiple antenna system.